Two principal capacitive sensing and measurement technologies are, currently employed in most touchpad and touchscreen devices. The first such technology is that of self-capacitance. Many devices manufactured by SYNAPTICS™ employ self-capacitance measurement techniques, as do integrated circuit (IC) devices such as the CYPRESS PSOC.™ Self-capacitance involves measuring the self-capacitance of a series of electrode pads using techniques such as those described in U.S. Pat. No. 5,543,588 to Bisset et al. entitled “Touch Pad Driven Handheld Computing Device” dated Aug. 6, 1996.
Self-capacitance may be measured-through the detection of the amount of charge accumulated on an object held at a givens voltage (Q=CV). Self-capacitance is typically measured by applying a known voltage to an electrode, and then using a circuit to measure how much charge flows to that same electrode. When external objects are brought close to the electrode, additional charge is attracted to the electrode. As a result, the self-capacitance of the electrode increases. Many touch sensors are configured such that the grounded, object is a finger. The human body is essentially a capacitor toe surface where the electric field vanishes, and typically has a capacitance of around 100 pF.
The second primary capacitive sensing and measurement technology employed in touchpad and touchscreen devices is that of mutual capacitance, where measurements are performed using a crossed grid of electrodes. See, for example, U.S. Pat. No. 5,861,875 to Gerpheide entitled “Methods and Apparatus for Data Input” dated Jan. 19, 1999. Mutual capacitance technology is employed in touchpad devices manufactured by CIRQUE.™ In mutual capacitance measurement, capacitance is measured between two conductors, as opposed to a self-capacitance measurement in which the capacitance of a single conductor is measured, and which may be affected by other objects in proximity thereto.
In a capacitive touchscreen, a user's finger represents an electrode connected to an electric field ground. Due to the widespread use of switching power supply converters to power capacitive touchscreens, the electric potential of a readout electronic ground terminal (or system-ground) may vary significantly in periodic or not strictly periodic fashion with respect to the voltage associated with the electric field ground. The electric field voltage or potential is assumed to be zero at infinity. Variation of the system ground voltage with respect to the electric field ground voltage may produce significant interference in mutual capacitance signals acquired from a capacitive touchscreen, which we refer to here generically as electromagnetic interference. Such interference. Can be considered as “noise” due to the generally unknown phase relationships between a readout sampling clock a swathing power supply converter block. Other contributions to electromagnetic interference can include “noise” coupling associated with undesired charges induced in the human body and the touchscreen by various ambient or environmental sources that may be asynchronous with respect to capacitive touchscreen drive signals.
What is needed is a capacitive touchscreen system capable of reducing or otherwise filtering the effects of electromagnetic interference.